Abstract Airborne lidar flights to determine air pollutant transport in the complex coastal environment of S California reveal details of vertical pollutant distributions, atmospheric boundary layer structure, and pollutant transport and diffusion unique to such an area. The lidar data show two (or more) layers of aerosols offshore each separated by relatively clear areas. Typically the top of the lower layer corresponds to the base of the temperature inversion over the water, and the upper layer lies within the temperature inversion. Little intermixing of the layers occurs unless onshore air flow carries both layers over the coastline. A short distance inland, afternoon heating and mechanical turbulence eliminate the layered structure. The upper layers which may be formed by sea-breeze undercutting, by convergence in complex terrain or at the land/sea interface, or by interaction of heated upslope flow with terrain features can flow offshore with existing winds or at night with the land-breeze. The origin of the layers aloft is an essential factor in determining the pollutant impact. Thus, transport and layer structure phenomena observed by lidar within the complex S California coastline environment may have an important bearing on any proposed land-use changes. Finally, the results of the study suggest the need for diffusion climatologies that take into account the frequency and duration of the wind regimes aloft in addition to the low-level thermal structure.